I think the alarm idea is good, but I think it should be you doing the waking, instead of the alarm for him. We did this with our oldest, now 5 1/2, where we would wake her at 11pm, and take her. She would go every time...and a lot. Now, she if she needs to go at night, her little body wakes her, and she just goes by herself.
Two reasons why it's better for you to take him: 1)He'll be groggy, and may need a little help getting there. 2) He sees that you are awake too, and he appreciates it.
My daughter has really only wet the bed maybe 3 times, but it was because she was afraid to ge up out of bed. Not afraid of monsters, but afraid that my husband and I would get mad at her for getting out of bed. Yah, not logical, but what kid is at that age? Now I have a night light that guides her to the bathroom when she needs to at night, and I remind her every night that it's ok for her to get up and go if she needs to.